254 Background: Treatment of prostate adenocarcinoma (PRAD) with androgen-receptor pathway inhibitors can drive resistance via the emergence of neuroendocrine prostate cancer (NEPC). These aggressive cancers can express targetable cell surface proteins such as DLL3. Blood-based assessment of target expression could identify patients who may benefit from emerging biologic therapies targeting these proteins. To this end, we tested whether we could detect DLL3 expression in NEPC from circulating chromatin using a liquid biopsy assay. Methods: Cell-free chromatin immunoprecipitation and sequencing (cfChIP-seq) was performed for H3K4me3 at Precede Biosciences for a total of 57 longitudinal plasma samples collected from 25 patients with diagnosed with NEPC. Tissue samples from the biopsy confirming NEPC were available for 11 patients. NEPC was diagnosed based on small cell morphology and expression of synaptophysin and chromogranin A. Immunohistochemistry staining of membranous DLL3 was performed on matched tissue slides collected at a median of 14 days from plasma draw (range: 7 days – 18 months) and quantified by a board-certified pathologist. H3K4me3 cfChIP-seq signal at the DLL3 gene promoter was compared between plasma samples from patients with vs. without membranous DLL3 expression on IHC. Results: In total, 8 tissue samples had positive membranous DLL3 staining (DLL3+) and 3 had negative membranous expression (DLL3-). H3K4me3 staining at the promoter of DLL3 was significantly elevated in patients with tissue DLL3+ compared to DLL3- (p = 0.024, Wilcoxon rank-sum test). H3K4me3 plasma cfChIP-seq accurately distinguished DLL3+ from DLL3- with an AUROC curve of 0.96. In addition, an exploratory analysis was performed to measure elevated H3K4me3 promoter signal at additional cell-surface target genes from plasma samples of patients with NEPC (Table). Elevated signal was defined as H3K4me3 signal at the gene promoter with modified Z-scores ≥ 4, when compared to a control cohort consisting of plasma samples from healthy volunteers and patients with PRAD. Conclusions: In this exploratory analysis, H3K4me3 plasma cfChIP-seq can identify NEPC with membranous expression of DLL3. More broadly, this assay could allow blood-based monitoring of target expression to guide treatment with emerging biologic therapies targeting cell surface proteins. Cell surface target expression prediction based on plasma H3K4me3 promoter signal. Cell surface target N of samples with elevated H3K4me3 promoter signal Percentage SLC44A4 18 32% DLL3 16 28% MUC1 16 28% NECTIN4 15 26% ERBB2 15 26% NCAM1 14 25%
Nawfal et al. (Sun,) studied this question.